Scientists have confirmed that fast-growing seaweed, which smothers marine life, has spread to California.

The weed has destroyed large swathes of the seabed in the Mediterranean over the past decade after escaping from an aquarium.

This strain has adapted to cold water, to the Mediterranean, and now California and it could spread elsewhere

Dr Jan Pawlowski

Growing at a rate of up to eight centimetres (3.5 inches) per day, the alga spreads like wildfire, depriving other plants and animals of vital sunlight and oxygen.

Marine experts are concerned that the seaweed could colonise other parts of the ocean, with devastating consequences for natural ecosystems.

The green fern-like seaweed is known as Caulerpa taxifolia.

Widespread invasion

It was accidentally released about 16 years ago in the Cote d'Azur and now covers around 6,000 hectares (15,000 acres) of seabed off France.

Outbreaks have also been recorded along the coast of Spain and Italy and as far afield as Croatia.

The weed has to be removed by divers

Now, two independent genetic assessments have proved that a similar invasive alga that is spreading off the coast of southern California is the same strain as the one that has wreaked havoc in the Mediterranean.

"This strain has adapted to cold water, to the Mediterranean, and now California and it could spread elsewhere," he added.

"In places where it becomes dominant, it changes the ecosystem."

Writing in the journal Nature, the team warns that the weed must be rapidly eradicated to prevent a new invasion.

Unwelcome stowaway

Scientists believe the seaweed first took up residence in the warm waters of the Cote d'Azur after escaping from the Oceanographic Institute of Monaco while the facility's fish tanks were being cleaned.